For the metal surface treatment, chromium-based surface treating agents such as chromate and phosphate/chromate agents are known in the art and are widely utilized even at the present. However, the recent trend of environmental regulations suggests an upcoming possibility to ban the chromium-based surface treating agents because of the toxicity and especially carcinogenicity of chromium. There is a desire to have a metal surface treating agent that is free of chromium and that achieves adhesion and corrosion resistance comparable to the chromate treating agents.
JP-A H11-29724 (Patent Document 1) proposes a non-chromate treating agent for rust prevention comprising a water-soluble resin, a thiocarbonyl-containing compound and a phosphate ion and optionally water dispersible silica. This system is improved in corrosion resistance, but lacks workability and adhesion to substrates. JP-A H08-073775 (Patent Document 2) discloses an acidic surface treating agent comprising at least two silane coupling agents. This system is short of corrosion resistance where high corrosion resistance and workability are required after the metal surface treatment.
In connection with these, JP-A 2001-316845 (Patent Document 3) discloses a non-chromate, metal surface treating agent comprising a silane coupling agent, water dispersible silica, and zirconium or titanium ion as essential components. Corrosion resistance and workability are improved, but coating to substrates and bond strength to an overcoat layer are still insufficient.
JP-A H10-60315 (Patent Document 4) discloses a surface treating agent for steel structures comprising a silane coupling agent having a specific functional group capable of reacting with an aqueous emulsion. In this case, only corrosion resistance to relatively mild tests like a humidity cabinet test is required. This agent is short of corrosion resistance as compared with the metal surface treating agent that withstands rigorous corrosion tests as contemplated in the present invention.
JP-A 2000-297093 (Patent Document 5) describes that an imidazole-containing organosilicon compound is used as a surface treating agent for metals. The surface treating agent is still unsatisfactory in corrosion resistance and deep drawing resistance. JP-A H06-279463 (Patent Document 6) describes that a benzotriazole-containing organosilicon compound is used as a surface treating agent for metals. This surface treating agent is likewise unsatisfactory in corrosion resistance and deep drawing resistance.
JP-A 2007-297648 (Patent Document 7) discloses a rust preventive surface treating agent comprising an aqueous emulsion, a compound having a trivalent transition metal ion coordinated with two molecules of β-diketone and two molecules of water, and a silane coupling agent. The surface treating agent is characterized in that the trivalent transition metal complex converts to a substantially insoluble compound on drying, exerting rust prevention and adhesion of an overcoat. The required level of corrosion resistance is not resistance against a rigorous environment as contemplated in the present invention. There is left room for further improvement.
Accordingly, there exists a desire to have a metal surface treating agent which in thin film form develops high levels of properties including corrosion resistance, work adhesion, coating and bond strength.